The invention relates to fuel cell systems.
A fuel cell can convert chemical energy to electrical energy by promoting a chemical reaction between two gases.
One type of fuel cell includes a cathode flow field plate, an anode flow field plate, a membrane electrode assembly disposed between the cathode flow field plate and the anode flow field plate, and two gas diffusion layers disposed between the cathode flow field plate and the anode flow field plate. A fuel cell can also include one or more coolant flow field plates disposed adjacent the exterior of the anode flow field plate and/or the exterior of the cathode flow field plate.
Each flow field plate has an inlet region, an outlet region and open-faced channels connecting the inlet region to the outlet region and providing a way for distributing the gases to the membrane electrode assembly.
The membrane electrode assembly usually includes a solid electrolyte (e.g., a proton exchange membrane, commonly abbreviated as a PEM) between a first catalyst and a second catalyst. One gas diffusion layer is between the first catalyst and the anode flow field plate, and the other gas diffusion layer is between the second catalyst and the cathode flow field plate.
During operation of the fuel cell, one of the gases (the anode gas) enters the anode flow field plate at the inlet region of the anode flow field plate and flows through the channels of the anode flow field plate toward the outlet region of the anode flow field plate. The other gas (the cathode gas) enters the cathode flow field plate at the inlet region of the cathode flow field plate and flows through the channels of the cathode flow field plate toward the cathode flow field plate outlet region.
As the anode gas flows through the channels of the anode flow field plate, the anode gas passes through the anode gas diffusion layer and interacts with the anode catalyst. Similarly, as the cathode gas flows through the channels of the cathode flow field plate, the cathode gas passes through the cathode gas diffusion layer and interacts with the cathode catalyst.
The anode catalyst interacts with the anode gas to catalyze the conversion of the anode gas to reaction intermediates. The reaction intermediates include ions and electrons. The cathode catalyst interacts with the cathode gas and the reaction intermediates to catalyze the conversion of the cathode gas to the chemical product of the fuel cell reaction.
The chemical product of the fuel cell reaction flows through a gas diffusion layer to the channels of a flow field plate (e.g., the cathode flow field plate). The chemical product then flows along the channels of the flow field plate toward the outlet region of the flow field plate.
The electrolyte provides a barrier to the flow of the electrons and gases from one side of the membrane electrode assembly to the other side of the membrane electrode assembly. However, the electrolyte allows ionic reaction intermediates to flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly.
Therefore, the ionic reaction intermediates can flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly without exiting the fuel cell. In contrast, the electrons flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly by electrically connecting an external load between the anode flow field plate and the cathode flow field plate. The external load allows the electrons to flow from the anode side of the membrane electrode assembly, through the anode flow field plate, through the load and to the cathode flow field plate.
Electrons are formed at the anode side of the membrane electrode assembly, indicating that the anode gas undergoes oxidation during the fuel cell reaction. Electrons are consumed at the cathode side of the membrane electrode assembly, indicating that the cathode gas undergoes reduction during the fuel cell reaction.
For example, when hydrogen and oxygen are the gases used in a fuel cell, the hydrogen flows through the anode flow field plate and undergoes oxidation. The oxygen flows through the cathode flow field plate and undergoes reduction. The specific reactions that occur in the fuel cell are represented in equations 1-3.
H2xe2x86x922H++2exe2x88x92xe2x80x83xe2x80x83(1)
1/2O2+2H++2exe2x88x92xe2x86x92H2Oxe2x80x83xe2x80x83(2)
H2+1/2O2xe2x86x92H2Oxe2x80x83xe2x80x83(3)
As shown in equation 1, the hydrogen forms protons (H+) and electrons. The protons flow through the electrolyte to the cathode side of the membrane electrode assembly, and the electrons flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly through the external load. As shown in equation 2, the electrons and protons react with the oxygen to form water. Equation 3 shows the overall fuel cell reaction.
In addition to forming chemical products, the fuel cell reaction produces heat. One or more coolant flow field plates are typically used to conduct the heat away from the fuel cell and prevent it from overheating.
Each coolant flow field plate has an inlet region, an outlet region and channels that provide fluid communication between the coolant flow field plate inlet region and the coolant flow field plate outlet region. A coolant (e.g., liquid de-ionized water or other low conductivity fluids) at a relatively low temperature enters the coolant flow field plate at the inlet region, flows through the channels of the coolant flow field plate toward the outlet region of the coolant flow field plate, and exits the coolant flow field plate at the outlet region of the coolant flow field plate. As the coolant flows through the channels of the coolant flow field plate, the coolant absorbs heat formed in the fuel cell. When the coolant exits the coolant flow field plate, the heat absorbed by the coolant is removed from the fuel cell.
To increase the electrical energy available, a plurality of fuel cells can be arranged in series to form a fuel cell stack. In a fuel cell stack, one side of a flow field plate functions as the anode flow field plate for one fuel cell while the opposite side of the flow field plate functions as the cathode flow field plate in another fuel cell. This arrangement may be referred to as a bipolar plate. The stack may also include monopolar plates such as, for example, an anode coolant flow field plate having one side that serves as an anode flow field plate and another side that serves as a coolant flow field plate. As an example, the open-faced coolant channels of an anode coolant flow field plate and a cathode coolant flow field plate may be mated to form collective coolant channels to cool the adjacent flow field plates forming fuel cells.
The invention relates to fuel cell systems. The fuel cell systems include a water recovery device that can transfer water from one or more fuel cell or fuel cell stack exhaust streams into a fuel cell or fuel cell stack inlet stream so that the water is recycled back into the fuel cell or fuel cell stack. This can improve fuel cell or fuel cell stack performance and/or efficiency.
In one aspect, the invention generally relates to a fuel cell system including a fuel cell stack and a water recovery device. The fuel cell stack has a cathode gas inlet, a cathode gas outlet and an anode gas outlet. The water recovery device has three different ports. One of the ports is in fluid communication with the cathode gas inlet so that at least a portion of a gas exiting this ports flows to the cathode gas inlet. Another port is in fluid communication with the cathode gas outlet so that at least a portion of a gas exiting the cathode gas outlet flows to this port. The third port is in fluid communication with the anode gas outlet so that at least a portion of a gas exiting the anode gas outlet flows to this port.
The water recovery can be, for example, an enthalpy wheel, a desiccant wheel or a sensible heat rotor.
The water recovery device can further include three additional ports arranged so that there is at least one of the additional ports between the three ports in fluid communication with the gas streams.
In some embodiments, the ports are arranged so that the port in fluid communication with the anode gas outlet is between the other two ports along the rotation path of the water recovery device.
In another aspect, the invention generally relates to a method of operating a fuel cell system that includes a fuel cell stack. The method includes flowing a cathode gas mixture to a port into a water recovery device, and flowing an outlet cathode gas into a different port in the water recovery device. The method also includes flowing an outlet anode gas into yet a different port in the water recover device.
The flow direction of the cathode gas mixture can be counter to or concurrent with the flow direction of the outlet cathode gas. The flow direction of the cathode gas mixture can be counter to or concurrent with the flow direction of the outlet anode gas.
In some embodiments, flowing the cathode gas mixture into the port forms a different cathode gas mixture different than the first cathode gas mixture. This different cathode gas mixture can have a higher water content than the cathode gas mixture that flows into the port.
The method can further include rotating the water recovery device along a rotation path so that the port into which the anode outlet gas flows is between the other two ports along the rotation path of the water recovery device.
The can further includes a cathode inlet gas and an anode inlet gas through at least a portion of the fuel cell stack to form water and electricity. The cathode inlet gas can be formed by transferring water from the outlet cathode gas mixture and/or the outlet anode gas mixture to the cathode gas mixture.
In a further aspect, the invention generally relates to a method of operating a fuel cell system including a fuel cell stack. The method includes flowing a gas mixture into a port in a water recovery device to form a different gas mixture and then flowing this gas mixture through at least a portion of the fuel cell stack to form an outlet gas. The method also includes flowing the outlet gas into a different port in the water recovery device, and flowing a different outlet gas into yet a different port in the water recover device.
In some embodiments, flowing the gas mixture into the first port forms a different gas mixture. The water content of the gas mixture so formed can be higher than that of the gas mixture that flows into the port.
In another aspect, the invention relates to a method of operating a fuel cell system that includes a fuel cell stack having a cathode gas outlet and an anode gas outlet. The method includes flowing a gas mixture (e.g., a gas mixture containing a cathode gas) from the cathode gas outlet into a water recovery device while also flowing a different gas mixture (e.g., a gas mixture containing an anode gas) from the anode gas outlet into the water recovery device without first mixing the first and second gas mixtures.
The can further include flowing a gas mixture from the water recovery device to a cathode gas inlet of the fuel cell stack.
One potential advantage of the invention is that, in certain embodiments, in addition to recovering water from the anode exhaust gas stream, the amount of energy used to humidify the reformer inlet gas stream is reduced, thereby improving the overall efficiency of the systems and methods.
Other features and advantages of the invention will be apparent from the description, the drawings and the claims.